by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

Sunday's quake is the strongest non-Alaska temblor to hit the U.S. so far this year, according to the U.S. Geological Survey (USGS). On average, about five quakes of this magnitude or stronger hit the U.S. each year, many in or near Alaska.

Alaska and California are the two most-earthquake-prone states in the nation, according to the USGS. Most earthquakes in California aren't newsworthy, however.

For example, "many earthquakes happen in the Bay Area every day," according to the Berkeley Seismological Laboratory. But most of them are too small to feel.

"On average, there is an earthquake that a few people will feel every two-three weeks and one that will be felt by many people every year," the laboratory reports on its website.

The worst quake in Northern California history was in 1906: The earthquake - centered in San Francisco - and resulting fires caused an estimated 3,000 deaths and $524 million in property loss.

In regard to Sunday's quake, "at this time there's no exact determination of the causative fault, though the Browns Valley section of the West Napa fault is suspected," the USGS tweeted Sunday morning.

As for the likelihood of strong aftershocks, as of early Sunday afternoon, "the probability of a strong and possibly damaging aftershock in the next seven days is approximately 45%," the Northern California Seismic System reported.

Something else to worry about: A study earlier this year in the journal Nature found a potential link between earthquakes and the drought in California. The study, led by geologist Colin Amos from Western Washington University, found that a lack of water in the San Joaquin Valley is decreasing the weight on the San Andreas Fault, which could lead to more earthquakes.